Android Factorial factorialDouble(final int n)

Description

Compute n!, the<a href="http://mathworld.wolfram.com/Factorial.html"> factorial</a> of n (the product of the numbers 1 to n), as a double .

License

Apache License

Parameter

Parameter	Description
n	Argument.

Exception

Parameter	Description
NotPositiveException	if n < 0.

Return

n!

Declaration

public static double factorialDouble(final int n)
        throws NotPositiveException 

Method Source Code

/*//from   w  w w.  j a v  a 2 s. c  om
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.math.BigInteger;
import java.util.concurrent.atomic.AtomicReference;
import org.apache.commons.math3.exception.MathArithmeticException;
import org.apache.commons.math3.exception.NotPositiveException;
import org.apache.commons.math3.exception.NumberIsTooLargeException;
import org.apache.commons.math3.exception.util.Localizable;
import org.apache.commons.math3.exception.util.LocalizedFormats;

public class Main{
    /** All long-representable factorials */
    static final long[] FACTORIALS = new long[] { 1l, 1l, 2l, 6l, 24l,
            120l, 720l, 5040l, 40320l, 362880l, 3628800l, 39916800l,
            479001600l, 6227020800l, 87178291200l, 1307674368000l,
            20922789888000l, 355687428096000l, 6402373705728000l,
            121645100408832000l, 2432902008176640000l };
    /**
     * Compute n!, the<a href="http://mathworld.wolfram.com/Factorial.html">
     * factorial</a> of {@code n} (the product of the numbers 1 to n), as a
     * {@code double}.
     * The result should be small enough to fit into a {@code double}: The
     * largest {@code n} for which {@code n! < Double.MAX_VALUE} is 170.
     * If the computed value exceeds {@code Double.MAX_VALUE},
     * {@code Double.POSITIVE_INFINITY} is returned.
     *
     * @param n Argument.
     * @return {@code n!}
     * @throws NotPositiveException if {@code n < 0}.
     */
    public static double factorialDouble(final int n)
            throws NotPositiveException {
        if (n < 0) {
            throw new NotPositiveException(
                    LocalizedFormats.FACTORIAL_NEGATIVE_PARAMETER, n);
        }
        if (n < 21) {
            return FACTORIALS[n];
        }
        return FastMath
                .floor(FastMath.exp(ArithmeticUtils.factorialLog(n)) + 0.5);
    }
    /**
     * Compute the natural logarithm of the factorial of {@code n}.
     *
     * @param n Argument.
     * @return {@code n!}
     * @throws NotPositiveException if {@code n < 0}.
     */
    public static double factorialLog(final int n)
            throws NotPositiveException {
        if (n < 0) {
            throw new NotPositiveException(
                    LocalizedFormats.FACTORIAL_NEGATIVE_PARAMETER, n);
        }
        if (n < 21) {
            return FastMath.log(FACTORIALS[n]);
        }
        double logSum = 0;
        for (int i = 2; i <= n; i++) {
            logSum += FastMath.log(i);
        }
        return logSum;
    }
}

Related

1. factorial(final int n)
2. factorialLog(final int n)